The present invention relates to a variable capacitor.
Conventionally, there have been proposed various types of variable capacitor, one of which is illustrated in FIG. 1 in cross-sectional view. The conventional variable capacitor shown in FIG. 1 includes a cylindrical cap member 1 made of an insulating material, open at one end 1a and provided at the other end with a small circular opening 1b, a disc shaped non-metallic elastic plate 2; a disc shaped terminal 3 having a leg 3a for an external electrical connection; a disc shaped rotatable electrode 4; a disc shaped dielectric plate 5; and a semi-circular stator electrode 6 having a leg 6a for an external electrical connection. A disc shaped stator plate 7 is tightly inserted into the cap member 1 in such a manner as to fill the portion of the interior of the cap member 1 below the plate 5. The cap member 1 is closed by a base plate 8 made of insulating material and having an opening 8a for the insertion of the leg 3a therethrough and an opening 8b for the insertion of the leg 6a therethrough.
The elastic plate 2 and the disc shaped terminal 3 have circular openings coaxial with the circular opening 1b while the rotatable electrode 4 has a groove 4a formed on its upper surface for receiving the tip of a screwdriver. When a screwdriver is inserted in groove 4a and rotated, the rotatable electrode 4 is rotated to change the position of a concavity 4b, formed on the lower surface of the rotatable electrode 4, thereby changing the electric capacitance between the rotatable electrode 4 and the stator electrode 6.
A variable capacitor of the above design can be damaged. By way of example, when a printed circuit is soldered to legs 3a and 6a, soldering flux tends to flow along one or both of the legs 3a and 6a into the interior of the cap 1 through the respective openings 8a and 8b.
In order to prevent such damage, it is conventional to provide a plate or a coating 9 of synthetic resin to the exterior surface of plate 8. This requires extra steps more components to construct the variable capacitor, as well as many adjustments. Thus, the conventional variable capacitor has a high manufacturing cost.
Moreover, since the dielectric plate 5 is bonded neither to the rotatable electrode 4, to the stator electrode 6 nor to the stator 7, the dielectric plate 5 can rotate together with the rotatable electrode 4. During its rotation, the dielectric plate 5 is susceptible to cracking or chipping-off.
In addition, since the dielectric plate 5 is manufactured separately from the stator electrode 6 and stator plate 7, the surface of the stator electrode 6 and plate 7 facing the dielectric plate 5 must be smooth. In view of this, the upper surface of the stator plate 7 and of the stator electrode 6, which contact the dielectric plate 5, should be finished smooth by means of lapping or a comparable technique. Moreover, since the stator plate 7 and the stator electrode 6 are made of different materials, the surface of the stator may be uneven at the joint between the stator plate 7 and the stator electrode 6 due to the difference between their thermal expansion coefficients.